acne: a serious, ongoing outbreak of pimples

adolescence: the stage of life between puberty and adulthood

ejaculation: when semen comes out of a penis

erection: when a penis gets hard

flaccid: when a penis stays soft

foreskin: skin over the head of a penis

glans: the head of a penis

hormones: chemicals in your body that send messages to the various parts of your body to "tell" them that it's time to mature.

jock itch: a skin infection caused by sweat and a lack of air circulation around the external genitalia

menstruation: the monthly shedding of a woman's uterine lining, also called a period

nocturnal emission: an ejaculation that occurs while a male adolescent sleeps; also called a wet dream

penis: the external male reproductive organ

pimple: a blemish caused by oil build-up in the pores of the skin

pituitary gland: a gland at the base of the brain that releases hormones

puberty: a time of physical and emotional change for adolescents and their bodies

pubic hair: hair that grows around the male and female genitals

scrotum: the soft sac of skin in males that holds the testes

semen: the white fluid that is pushed out from the penis and carries sperm

sperm: the male cell that joins a female egg to make a baby

testicles: male organs that produce hormones and sperm, also called testes

testosterone: male hormone

tubules: small tubes in the testicles where sperm is made

vagina: a female reproductive organ